filter only works for class methods

[TomW1605]

the filter function passes on all but the first argument. looking at the testing code that makes sense if the function being filtered is a class function since it removes the self argument that is added implicity. however it breaks when it is not a class function and so does not have the self argument.

this can be worked around by adding a dummy argument to the function and all its calls but that is very messy. is there some check that could be done to only remove that argument when the function is a class method and leave it if not?